Diversity reception device and diversity reception method

ABSTRACT

The circuit size of a diversity receiver for an orthogonal frequency division multiplexing signal is reduced, and the diversity effect is increased, by providing a power ratio comparator that calculates a difference value as a ratio of powers derived from channel estimation results for subcarrier components received from two antennas ( 11 ), ( 21 ) and compares the calculated difference value with a predetermined threshold, and a selective/equal gain combining selector ( 33 ) that outputs one of the received demodulated signals when the comparison result indicates that the calculated difference value is greater than the threshold value.

FIELD OF THE INVENTION

The present invention relates to a diversity receiver having a pluralityof demodulation paths, and to its receiving method.

BACKGROUND ART

The diversity receivers found in the prior art first compare estimatedreceived power values of the carrier waves of the received signals oneach of two demodulation paths at each point in time, and select andoutput the received signal with the larger estimated value; this isgenerally known as selection diversity (also referred to below as theselection system or selective diversity). That is, of the two receivedsignals at each point in time, they selectively output the receivedsignal with the better reception conditions, and do not use the receivedsignal with the inferior reception conditions. At each point in time,accordingly, they cannot obtain better receiving performance than theindividual received power obtained from one of the received signals inthe two demodulation paths.

To improve the receiving performance further, combining the two receivedsignals has been contemplated.

A type of diversity receiver is known that employs a maximal ratiocombining diversity system by providing circuitry that calculates aratio of power levels (estimated power values) of a pair of receivedsignals on a pair of demodulation paths (or demodulated signals obtainedby demodulating the received signals), generates weighting coefficientsaccording to the calculated power ratio, and multiplies the receivedsignals by the weighting coefficients to create a weighted combination.

It is known, as shown in “Improvement of terrestrial digital TVbroadcasting performance by diversity receiving” by Takashi Seki, etal., Technology Report from Image Information Media Academy, May 25,2001, Vol. 25, No. 34, pp. 1 to 6, ROFT2001-54 (May, 2001), that amaximal ratio combining diversity receiver can not only mitigatemultipath distortion, as do diversity receivers using the selectiondiversity system, but also improve transmission characteristics withrespect to thermal noise, and can further improve the instantaneouscarrier-to-noise ratio (also referred to simply as the CNR below).

Equal gain combining diversity receivers are another example of adiversity system in which a pair of received signals on a pair ofdemodulation paths are combined to improve receiving performance. Equalgain combining diversity always combines a pair of received signals withequal gain, so that regardless of the power levels (estimated powervalues) of the received signals on the pair of demodulation paths, theaverage value of the received signals on the demodulation paths isalways output as the combined signal. It is known that equal gaincombining diversity produces a larger diversity effect than selectiondiversity and a smaller diversity effect than maximal ratio combiningdiversity. By contrast, when the difference between the received signalson the pair of demodulation paths (or the demodulated signals obtainedby demodulation of the received signals) or between the CNRs of thereceived signals increases, the receiving performance of equal gaincombining diversity may fall below that of selection diversity.

Among conventional diversity receivers, selection diversity receivers,for example, can operate with small circuitry because they simply useone of the received signals on the pair of demodulation paths, but therehas been a problem in that it is difficult to improve their receivingperformance.

Although equal gain combining diversity receivers require only simpleequalizers to be added and can accordingly operate with comparativelysmall circuitry, and although they can provide better reception thanwith selection diversity, there has been a problem in that theirreception cannot be improved over that of maximal ratio combiningdiversity. There has also been a problem in that as the differencebetween the received signals on the pair of demodulation pathsincreases, the receiving performance of equal gain combining diversityreceivers is degraded.

Maximal ratio combining diversity receivers can provide better receivingperformance than selection or equal gain combining diversity receivers,but there has been a problem in that they require circuitry forgenerating weighting coefficients according to the (estimated) receivedsignal power ratio and further multiplying the received signal powers bythe weighting coefficients, resulting in larger circuit scale.

The present invention is intended to solve problems such as those above,and has the object of providing a diversity receiver with a smallcircuit scale in which the receiving performance can be improved to alevel near that of a maximal ratio combining diversity receiver.

DISCLOSURE OF THE INVENTION

The diversity receiver of the present invention has: a plurality ofdemodulation paths for demodulating received signals and outputtingdemodulated signals; a power ratio comparator for calculating a powerratio from a first power corresponding to a first received signal on oneof the demodulation paths and a second power corresponding to a secondreceived signal on another one of the demodulation paths, and comparingthe power ratio with a predetermined threshold value; a signal selectorfor selecting one of the demodulated signals output from the pluralityof demodulation paths and outputting the selected demodulated signal; anequal-gain signal combiner for combining the demodulated signals outputfrom the plurality of demodulation paths with predetermined gains, andoutputting a combined demodulated signal; and a demodulated signaloutput unit for outputting one of the demodulated signals, either theselected demodulated signal or the combined demodulated signal,responsive to the result of the comparison in the power ratiocomparator.

The diversity receiving method of the present invention adaptivelyswitches between selection diversity and equal gain combining diversityfor each subcarrier component according to power values of the receivedsignals on the demodulation paths, so in comparison with conventionaldiversity receiving methods using only selection diversity or only equalgain combining diversity, it can provide a larger diversity effect andimproved receiving performance, and a diversity receiver producing alarge diversity effect can be implemented with less circuitry than whenmaximal ratio combining diversity is practiced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a diversity receiver in a firstembodiment of the invention.

FIG. 2 is a drawing showing a scattered pilot, which is a known pilotsubcarrier component inserted periodically among the Fourier-transformedOFDM subcarriers.

FIG. 3 is a drawing simulating the CNR's in the selection system, equalgain combining system, and maximal ratio combining system.

FIG. 4 is a drawing simulating the CNRs in the adaptive combining systemand maximal ratio combining system.

FIG. 5 is a block diagram illustrating a diversity receiver in a secondembodiment of the invention.

FIG. 6 is a block diagram illustrating a diversity receiver in a thirdembodiment of the invention.

FIG. 7 is a block diagram illustrating a diversity receiver in a fourthembodiment of the invention.

FIG. 8 is a block diagram illustrating a diversity receiver in a fifthembodiment of the invention.

FIG. 9 is a block diagram illustrating a diversity receiver in a sixthembodiment of the invention.

FIG. 10 is a block diagram illustrating a diversity receiver in aseventh embodiment of the invention.

FIG. 11 is a block diagram illustrating a diversity receiver in aneighth embodiment of the invention.

FIG. 12 is a block diagram showing the structure of the firstpre-combination error correction unit in FIG. 11.

FIG. 13 is a flow diagram showing an example of the operation of themain parts of the diversity receiver in FIG. 11.

FIG. 14 is a block diagram illustrating a diversity receiver in a ninthembodiment of the invention.

FIG. 15 is a flow diagram showing an example of the operation of themain parts of the diversity receiver in FIG. 14.

BEST MODE OF PRACTICING THE INVENTION

In the following descriptions of the embodiments, the case in which anorthogonal frequency division multiplexing (OFDM) signal is received inthe instant diversity receiver will be described. OFDM transmissiontechnology and diversity technology will be described before thedescription of the embodiments.

OFDM transmission technology (for transmitting and receiving) transmitsinformation modulated onto a multiplexed plurality of subcarriers havingmutually orthogonal frequencies and performs the reverse process at thereceiving end to demodulate the signal; practical use of thistransmitting and receiving technology is advancing, particularly in thebroadcasting and communication fields.

In OFDM transmission, the transmitter first assigns the data to betransmitted to a plurality of subcarriers, and modulates each subcarrierdigitally by a system such as QPSK (Quadrature Phase Shift Keying), QAM(Quadrature Amplitude Modulation), or DQPSK (Differential EncodedQuadrature Phase Shift Keying). Additional information relating totransmission parameters and transmission control, and a continuous pilotcarrier component modulated with known data, are modulated onto aparticular subcarrier using DBPSK (Differential Binary Phase ShiftKeying) or BPSK (Binary Phase Shift Keying); after these aremultiplexed, the OFDM signal is converted to a desired frequency andtransmitted.

More specifically, in transmission, the data to be transmitted aremapped onto the subcarriers according to the modulation system thereof,and an inverse discrete Fourier transform is performed. Next, after theinverse discrete Fourier transform, the last part of the signal iscopied to the beginning of the signal. This part is referred to as theguard interval; this enables the signal to be received without symbolinterference at the receiving end even if there is a delayed signalhaving a delay time equal to or less than the guard interval.

Because all of the subcarriers in the OFDM system possess mutualorthogonality, the transmitted data can be recovered correctly if thesubcarrier frequencies are correctly recovered at the receiving end.When the subcarrier frequencies at the receiving end include error withrespect to the actual frequencies, however, intercarrier interferenceoccurs, the probability of incorrect recovery of the transmitted dataincreases, and transmission characteristics are degraded. Accordingly,the accuracy with which the subcarrier frequencies can be recovered atthe receiving end is a critical issue in an OFDM system.

A demodulator that receives an OFDM signal orthogonally demodulates thecomplex digital OFDM signal that is generally input, converting itsfrequency to the baseband, removes the guard intervals to obtain atime-domain signal, and Fourier-transforms the time-domain signal toobtain a frequency-domain signal, which is then detected and therebydemodulated.

In an OFDM system, each subcarrier carries transmitted data mappedaccording to a modulation system such as QPSK or multilevel QAM; knownpilot carrier signals are inserted among the subcarriers periodically inthe frequency and time directions. In the Japanese terrestrial digitalTV broadcasting system, for example, a scattered pilot is insertedperiodically; the OFDM receiver estimates channel characteristics on thebasis of the scattered pilot to demodulate the subcarriers.

Diversity technology uses a plurality of demodulation paths (at leasttwo paths) as described above from respective antennas to respectiveOFDM demodulators, thereby obtaining higher receiving performance thanwhen a single demodulation path is used. When signals are received inadverse transmission environments caused by multipath or Rayleigh-fadingchannels, diversity technology, by effecting spatial diversity,generally reduces the error rate after signal demodulation and improvesthe receiving performance.

FIRST EMBODIMENT

FIG. 1 is a block diagram illustrating the diversity receiver in thefirst embodiment.

As OFDM signal demodulation paths, the diversity receiver has twodemodulation paths: demodulation path A and demodulation path B.Demodulation path A has a first antenna 11, a first tuner 12, a firstAGC (Automatic Gain Control) unit 13, a first ADC (analog-to-digitalconverter) 14, and a first OFDM demodulator 15. Demodulation path B hasa second antenna 21, a second tuner 22, a second AGC unit 23, a secondADC 24, and a second OFDM demodulator 25.

In the diversity receiver illustrated in FIG. 1, the first antenna 11and second antenna 21 receive wireless signals that have been modulatedfor transmission. The first tuner 12 and second tuner 22 convert thefrequency of the received wireless signals to a predetermined frequencyband.

The first AGC unit 13 and second AGC unit 23 adjust the gain levels ofthe frequency-converted analog signals. The gain level adjustmentperformed by the first AGC unit 13 and second AGC unit 23 producesoptimal signal levels in the first and second demodulators 46, 56 in thefollowing stage. Adjustment of the gain by the AGC circuits 13, 23 ispreferable because in general the signal power of the received signalsinput from the antennas 11, 21 varies due to, for example, the antennagain and channel conditions.

The first ADC 14 and second ADC 24 convert the frequency-converted andgain-adjusted analog signals to digital signals, outputting a firstreceived signal and a second received signal to the first OFDMdemodulator 15 and second OFDM demodulator 25, respectively.

The first OFDM demodulator 15 and second OFDM demodulator 25 demodulatethe first received signal and the second received signal and outputrespective digital demodulated signals.

Signals corresponding to power (referred to as estimated power P_(es)below) in estimated channel values calculated for each subcarrier in thereceived signals on demodulation paths A and B are input from the firstOFDM demodulator 15 and second OFDM demodulator 25 to a power ratiocomparator 31.

The power ratio comparator 31 decides which of the estimated powervalues P_(es) is larger: the estimated power value P_(es) _(—) _(A) ofdemodulation path A or the estimated power value P_(es) _(—) _(B) ofdemodulation path B. It also compares an estimated power ratio P_(es)_(—) _(R) obtained by dividing the larger of the two estimated powervalues P_(es) _(—) _(A) and P_(es) _(—) _(B) by the smaller of thesevalues with a predetermined threshold (referred to in the firstembodiment as the power ratio threshold) for each subcarrier, andoutputs a signal indicating the result of the comparison to theselective/equal gain combining selector 33.

More specifically, if the estimated power ratio P_(es) _(—) _(R) issmaller than the power ratio threshold value, the power ratio comparator31 outputs to the selective/equal gain combining selector 33 a signalindicating that a demodulated signal obtained by an equal-gain signalcombiner 62, described later, will be output. On the other hand, if theestimated power ratio P_(es) _(—) _(R) is larger than the power ratiothreshold, the power ratio comparator 31 outputs to the selective/equalgain combining selector 33 a signal indicating that the demodulatedsignal with the larger of the two estimated power values P_(es) _(—)_(A), P_(es) _(—) _(B) will be selected by the signal selector 61,described later.

In other words, the power ratio comparator 31 calculates the power ratioP_(es) _(—) _(R) from a first estimated power value which is a firstpower corresponding to the first received signal on demodulation path Aand a second estimated value which is a second power corresponding tothe second received signal on demodulation path B, and compares thepower ratio P_(es) _(—) _(R) with a predetermined threshold value (powerratio threshold).

According to the signal received from the power ratio comparator 31, theselective/equal gain combining selector 33 decides whether to output ademodulated signal (hereinafter also referred to as a selecteddemodulated signal) that is obtained by selecting one of the twodemodulated signals output from the first OFDM demodulator 15 and thesecond OFDM demodulator 25 (selection diversity) or a demodulated signal(hereinafter also referred to as a combined demodulated signal) that isobtained by combining both the demodulated signals with equal gain(equal gain combining diversity). Accordingly, in the diversity receiveraccording to the first embodiment, a resultant demodulated signal isoutput by the selective/equal gain combining selector 33. Theselective/equal gain combining selector 33 thus functions as thedemodulated signal output unit of the diversity receiver.

In other words, based on the output of the power ratio comparator 31,the selective/equal gain combining selector 33 outputs the single outputfrom the first demodulator 46, the single output from the seconddemodulator 56, or a combined output obtained by combining the outputsfrom the first demodulator 46 and the second demodulator 56 with equalgain.

Accordingly, the selectively combined or equal gain combined signaloutput from the selective/equal gain combining selector 33 is a signalobtained by adaptively selecting either a demodulated signal obtained byselecting one of the demodulated signals corresponding to the first andsecond received signals for each subcarrier component or a demodulatedsignal that is combined with equal gain according to the estimated powerratio P_(es) _(—) _(R) of the received signals on demodulation paths Aand B; the diversity effect of the two demodulation paths A and Breduces the error rate of the modulated signal.

The error correction unit 34 performs error correction on theselectively combined or equal-gain combined signal output from theselective/equal gain combining selector 33 and outputs the correcteddemodulated signal.

Next, the internal structure of the first OFDM demodulator 15 and thesecond OFDM demodulator 25 will be described. GI removers 41, 51 areprovided for eliminating guard intervals (GI's) in the first OFDMdemodulator 15 and the second OFDM demodulator 25, respectively. Thefirst GI remover 41 takes the first received signal as input andrecovers the OFDM symbol timing to eliminate the guard intervals addedto the first received signal; the second GI remover 51 takes the secondreceived signal as input and recovers the OFDM symbol timing toeliminate the guard intervals added to the second received signal.

A first FFT unit 42 and second FFT unit 52 convert input time domainsignals by the Fast Fourier Transform (referred to as FFT below) tooutput frequency domain signals. The frequency domain signals correspondto the subcarrier components of the first received signal and the secondreceived signal.

A first channel estimator 43 and second channel estimator 53 extractpilot carrier components included in the frequency domain signals outputfrom the first FFT unit 42 and second FFT unit 52 to estimate thechannel characteristics of the signals received from antennas 11 and 21.For example, in the Japanese terrestrial wave digital TV broadcastingsystem, scattered pilots are inserted periodically as shown in FIG. 2,and are used by TV receivers to estimate channel characteristics fordemodulation of the carrier waves. A general channel estimation method,for example, divides each of the extracted scattered pilots by its knowndata and interpolates the results in the time and frequency directions,thereby enabling estimation of the channel characteristics for eachsubcarrier component.

A first estimated power value calculator 44 and second estimated powervalue calculator 54 calculate the estimated power P_(es) _(—) _(A),P_(es) _(—) _(B) on the channels estimated for each subcarrier in thechannel estimators 43, 53, and output the results to a first demodulator46, a second demodulator 56, and the power ratio comparator 31. Asdescribed above, in this embodiment, since the received signals are OFDMsignals modulated by the OFDM modulating system, the pilot signals(pilot subcarrier components) included in the OFDM signals are used asreference signals by the channel estimators 43, 53 to estimate thechannel characteristics; the estimated power corresponding to each ofthe results is calculated by the estimated power value calculators 44,54; the estimated power values are input to the power ratio comparator31 as the received power; the power ratio comparator 31 calculates thepower ratio of the power values; the calculated power ratio is comparedwith the predetermined threshold; the result of the comparison is outputto the selective/equal gain combining selector 33.

A first demodulator 46 and second demodulator 56 demodulate eachsubcarrier component by dividing the frequency domain signals outputfrom the FFT units 42, 52 by signals corresponding to the channelestimation results output from the channel estimators 43, 53. Thisoperation is equivalent to multiplying the frequency domain signal bythe complex conjugate signal of the channel estimation results and thendividing the result by the power value of the estimated channelcharacteristic. More specifically, the first demodulator 46 multipliesthe output of the first FFT unit 42 by the complex conjugate signal ofthe output of the first channel estimator 43 and divides the result bythe first estimated power value P_(es) _(—) _(A). The second demodulator56 multiplies the output of the second FFT unit 52 by the complexconjugate signal of the output of the second channel estimator 53 anddivides the result by the second estimated power value P_(es) _(—) _(B).

Next, the internal structure of the selective/equal gain combiningselector 33 will be described. The selective/equal gain combiningselector 33 has a signal selector 61 and an equal-gain signal combiner62. The signal selector 61 outputs a signal by the selection diversitysystem; more specifically, it selects either the first demodulatedsignal output from the first OFDM demodulator 15 or the seconddemodulated signal output from the second OFDM demodulator 25 andoutputs the selected signal as the selected demodulated signal.

The equal-gain signal combiner 62 outputs a signal by the equal-gaincombined diversity system; it combines the first demodulated signaloutput from the first OFDM demodulator 15 and the second demodulatedsignal output from the second OFDM demodulator 25 with equal gain andoutputs the result as a combined demodulated signal.

In the selective/equal gain combining selector 33, switching between thesignal selector 61 and equal-gain signal combiner 62 may be performed byproviding a switching means or other equivalent means. The receivedsignal used in this embodiment is an OFDM signal, which includes aplurality of subcarrier components; a demodulated signal output unit 68outputs either the selectively demodulated signal or the combineddemodulated signal for each subcarrier component. The signal resultingfrom the comparison by the power ratio comparator 31 is obtained fromthe result of a comparison of the power ratio with a threshold valuethat is determined under the condition that thereceived-power-to-noise-power ratio of the demodulated signal obtainedby equal-gain combining of the plurality of demodulated signals mustequal the maximum of the received-power-to-noise-ratios of the pluralityof demodulated signals.

The selective/equal gain combining selector 33 outputs to the errorcorrection unit 34 either the demodulated signal obtained by the signalselector 61 or the demodulated signal obtained by the equal-gain signalcombiner 62 as a selectively combined or equal-gain combined signal,responsive to the output from the power ratio comparator 31.

The method of determining from the output of the power ratio comparator31 whether to use the demodulated signal from the signal selector 61 orthe demodulated signal from the equal-gain signal combiner 62 as theselectively combined or equal-gain combined signal will be described.

In general, when two antennas, the first antenna 11 and second antenna21, are used to perform spatially selective diversity, or selectiondiversity, the instantaneous received-power-to-noise-power ratio of thefinally output demodulated signal (CNR)_(SC) is expressed by equation 1below.(CNR)_(SC)=max[(CNR)_(A), (CNR)_(B)]  (1)

(CNR)_(A), (CNR)_(B), and (CNR)_(SC) are the carrier-to-noise ratios ofthe subcarrier on demodulation path A, the subcarrier on demodulationpath B, and the selectively combined signal, respectively; the functionmax[X1, X2] selects and outputs the larger of X1 and X2. It is assumedthat the two antennas receive signals with equal noise power. Under thisassumption, the amounts of power corresponding to the subcarriers areproportional to the values of (CNR)_(A) and (CNR)_(B).

The carrier-to-noise ratio of the final demodulated signal output by theequal gain combining diversity system, (CNR)_(EGC), is expressed byequation 2 below.

$\begin{matrix}{({CNR})_{EGC} = {\frac{1}{2}\left( {\sqrt{({CNR})_{A}} + \sqrt{({CNR})_{B}}} \right)^{2}}} & (2)\end{matrix}$

If the carrier-to-noise ratio of the final demodulated signal output bythe maximal ratio combining diversity system is (CNR)_(MRC), (CNR)_(MRC)is expressed by equation 3 below.(CNR)_(MRC)=(CNR)_(A)+(CNR)_(B)  (3)

FIG. 3 is a graph showing computer simulation results of thecarrier-to-noise ratios for selection diversity, equal gain combiningdiversity, and maximal ratio combining diversity, based on equations 1,2, and 3. (CNR)_(A) becomes CNR₁ in FIG. 3, and (CNR)_(B) becomes CNR₂;the graph shows the carrier-to-noise ratios of the final outputdemodulated signals for each type of diversity if CNR₁ is fixed at 20 dBand CNR₂ is varied from 0 dB to 40 dB. The circles indicate thecarrier-to-noise ratio for selection diversity; the squares indicate thecarrier-to-noise ratio for equal gain combining diversity; the starsindicate the carrier-to-noise ratio for maximal ratio combiningdiversity.

It can be seen from FIG. 3 that the closer the carrier-to-noise ratioCNR₂ of the second received signal is to the carrier-to-noise ratio CNR₁of the first received signal (fixed at 20 dB in FIG. 3), the less thediversity effect of selection diversity becomes in comparison withmaximal ratio combining diversity. For equal gain combining diversity,conversely, the further CNR₂ is from CNR₁, the less the diversity effectbecomes in comparison with maximal ratio combining diversity.

It can accordingly be seen that the diversity effect can be improved bychoosing either the selected demodulated signal obtained by the signalselector 61 or the combined demodulated signal obtained by theequal-gain signal combiner 62 as the selectively combined or equal gaincombined signal. The choice can be made between using the selecteddemodulated signal or the combined demodulated signal as the selectivelycombined or equal gain combined signal by setting boundaries where thecarrier-to-noise ratio obtained by selection diversity becomes equal tothe carrier-to-noise ratio obtained by equal gain combining diversity.That is, the switchover between the selected demodulated signal and thecombined demodulated signal can be made according to formula 4 below.The term 3+2√2 in formula 4 is the value of the ratio of thecarrier-to-noise ratios at which the left side of equation 1 becomesequal to the left side of equation 2. Diversity carried out by switchingbetween the selected demodulated signal and combined demodulated signalaccording to the condition given in formula 4 will also be referred toas adaptive combining diversity in the description below.

$\begin{matrix}{{{Combining}\mspace{14mu}{method}} = \left\{ \begin{matrix}{{{equal}\mspace{14mu}{gain}},} & {{{when}\mspace{31mu}\frac{\max\left\lbrack {({CNR})_{A},({CNR})_{B}} \right\rbrack}{\min\left\lbrack {({CNR})_{A},({CNR})_{B}} \right\rbrack}} \leq {3 + {2\sqrt{2}}}} \\{{selection},} & {otherwise}\end{matrix} \right.} & (4)\end{matrix}$

When the selected demodulated signal is selected according to formula 4,in the signal selector 61, it suffices to select one of the demodulatedsignals output from demodulation paths A and B according to thecondition given in formula 5 below,

$\begin{matrix}{{{Combined}\mspace{14mu}{signal}} = \left\{ \begin{matrix}{S_{A},} & {{{when}\mspace{31mu}({CNR})_{A}} \geq ({CNR})_{B}} \\{S_{B},} & {otherwise}\end{matrix} \right.} & (5)\end{matrix}$

In formula 5, S_(A) denotes the-demodulated signal input to theselective/equal gain combining selector 33 through demodulation path A,that is, the first demodulated signal, and S_(B) denotes the demodulatedsignal input to the selective/equal gain combining selector 33 throughdemodulation path B, that is, the second demodulated signal.

Adaptive diversity is a system that selects either selection diversityor equal gain combining diversity adaptively to increase thecarrier-to-noise ratio corresponding to the final output demodulatedsignal.

FIG. 4 shows computer simulated results of carrier-to-noise ratios whenadaptive combining diversity and maximal ratio combining diversity areused. CNR₁ is fixed at 20 dB, and CNR₂ is varied from 0 dB to 40 dB. Thetriangles in FIG. 4 indicate the carrier-to-noise ratio obtained byadaptive combining diversity; the stars indicate the carrier-to-noiseratio obtained by maximal ratio combining diversity.

In FIG. 4, the adaptive diversity processing was switched at the pointsat which the carrier-to-noise ratio obtained by selection diversity andthe carrier-to-noise ratio obtained by equal gain combining diversitybecame equal (near 12 dB and 28 dB on the scale on the horizontal axisin FIG. 4). The output was accordingly obtained by selection diversitywhen CNR₂ was greater than 0 dB and less than about 12 dB, and when CNR₂was greater than about 28 dB, and by equal gain combining diversity inthe interval from about 12 dB to about 28 dB. That is, selectiondiversity and equal gain combining diversity were switched at thresholdvalues of ±8 dB from a center value of 20 dB on the horizontal scale inFIG. 4.

The value of CNR₂ at 12 dB on the horizontal scale under the conditionsin FIG. 4,CNR ₂=10×log₁₀(10^((20/10))/(3+2√2)) dBcorresponds to the case in which the value of the CNR ratio in formula 4is:(CNR)_(A)/(CNR)_(B)=3+2√2The value of CNR₂ at 28 dB on the horizontal scale under the conditionsin FIG. 4,CNR ₂=10×log₁₀(10^((20/10))/(3+2√2)) dBcorresponds to the case in which the value of the CNR ratio in formula 4is:(CNR)_(B)/(CNR)_(A)=3+2√2

From the above, the threshold value in the power ratio comparator 31 isdetermined from conditions under which the received-power-to-noise-powerratio of the demodulated signal obtained by combining a plurality ofdemodulated signals with equal gain becomes equal to the maximalreceived-power-to-noise-power ratio among thereceived-power-to-noise-power ratios corresponding to each of theplurality of demodulated signals. The signal selector 61 selects andoutputs the demodulated signal having the maximalreceived-power-to-noise-power ratio among thereceived-power-to-noise-power ratios corresponding to each of thedemodulated signals output from demodulation paths A and B.

It can be seen from FIG. 4 that by using the adaptive combiningdiversity described above in the first embodiment, the diversity effectcan be improved, compared with the use of selection diversity or equalgain combining diversity alone, the effect becoming substantially thesame as when maximal ratio combining diversity is used.

Adaptive combining diversity can accordingly be carried out by input ofthe power ratio threshold value corresponding to the estimated powerratio P_(es) _(—) _(R), using the correspondence relationship betweenthe estimated power values P_(es) _(—) _(A), P_(es) _(—) _(B) and(CNR)_(A), (CNR)_(B).

Thus, because the diversity receiver in the first embodiment isstructured to output either a selected demodulated signal or a combineddemodulated signal as the selectively combined or equal gain combinedsignal for each subcarrier component adaptively, in such a way as toincrease the carrier-to-noise ratio of the selectively combined or equalgain combined signal output from the selective/equal gain combiningselector 33, it becomes possible to increase the diversity effect ascompared with conventional diversity receivers using only selectiondiversity or only equal gain combining diversity. The receivingperformance of the diversity receiver can also be improved. Thediversity receiver in the first embodiment can also increase thediversity effect with a smaller circuit scale than when maximal ratiocombining diversity is practiced.

SECOND EMBODIMENT

The first embodiment provides a structure in which adaptive combiningdiversity is carried out using power estimates P_(es) _(—) _(A), P_(es)_(—) _(B) output from the estimated power value calculators 44, 54. Inthe second embodiment, the power of the subcarrier components iscalculated from the signal output after the Fourier transform, andadaptive combining diversity is carried out using the calculated result.In the description below, subcarrier component power is also referred toas subcarrier power, and the value of the subcarrier power is alsoreferred to as a subcarrier power value.

FIG. 5 is a block diagram showing a diversity receiver in the secondembodiment.

The structure of the diversity receiver in FIG. 5 is the same as shownin FIG. 1 in the first embodiment, except for the first OFDM demodulator15 a, second OFDM demodulator 25 a, power ratio comparator 31 a, firstsubcarrier power calculator 45, and second subcarrier power calculator55, and except that there are no output connections from the firstestimated power value calculator 44 and second estimated power valuecalculator 54 to the power ratio comparator 31 a.

The operation of the diversity receiver in the second embodiment will bedescribed below. Descriptions of structures that are the same as in thefirst embodiment will be omitted.

The first subcarrier power calculator 45 in the first OFDM demodulator15 a receives the frequency domain signal on demodulation path A, andcalculates and then outputs the subcarrier power P_(c) _(—) _(A) of thefrequency domain signal. Similarly, the second subcarrier powercalculator 55 in the second OFDM demodulator 25 a receives the frequencydomain signal on demodulation path B, and calculates and then outputsthe subcarrier power P_(c) _(—) _(B) of the frequency domain signal.

The power ratio comparator 31 a receives subcarrier power values P_(c)_(—) _(A), P_(c) _(—) _(B) and a predetermined threshold valuecorresponding thereto. In the second embodiment, this threshold value,to which a power ratio obtained from the above power values is compared,will be referred to as the power ratio threshold value, as in the firstembodiment.

The power ratio comparator 31 a determines which of the two subcarrierpower values, P_(c) _(—) _(A), P_(c) _(—) _(B) is larger. It furthermorecompares the subcarrier power ratio P_(c) _(—) _(R) obtained by dividingthe larger one of the two subcarrier power values P_(c) _(—) _(A) andP_(c) _(—) _(B) by the smaller one with the power ratio threshold value,and outputs a signal corresponding to the result of the comparison foreach subcarrier to the selective/equal gain combining selector 33.

More specifically, when the subcarrier power ratio P_(c) _(—) _(R) issmaller than the power ratio threshold value, the power ratio comparator31 a outputs to the selective/equal gain combining selector 33 a signalindicating that the demodulated signal obtained in the equal-gain signalcombiner 62 is to be output. Conversely, when the subcarrier power ratioP_(c) _(—) _(R) is larger than the power ratio threshold value, thepower ratio comparator 31 a outputs to the selective/equal gaincombining selector 33 a signal indicating that the demodulated signalcorresponding to the larger of the two subcarrier power values P_(c)_(—) _(A) and P_(c) _(—) _(B) is to be selected.

Responsive to the signal received from the power ratio comparator 31 a,the selective/equal gain combining selector 33 selects either selectiondiversity, in which either the demodulated signal from the first OFDMdemodulator 15 a or the demodulated signal from the second OFDMdemodulator 25 a is selected and output, or equal gain combiningdiversity, in which a demodulated signal obtained by combining the twodemodulated signals from the first OFDM demodulator 15 a and second OFDMdemodulator 25 a with equal gain is selected and output.

That is, according to the output of the power ratio comparator 31 a, theselective/equal gain combining selector 33 outputs the output signalfrom the first demodulator 46 alone, the output signal from the seconddemodulator 56 alone, or a combined output signal obtained by combiningthe above output signals.

Accordingly, the selectively combined or equal gain combined signaloutput from the selective/equal gain combining selector 33 is a signalobtained for each subcarrier component by adaptively selecting either ademodulated signal corresponding to one of the subcarrier power valuesP_(c) _(—) _(A) and P_(c) _(—) _(B) of the pair of received signals orthe demodulated signal obtained by combining the first demodulatedsignal and the second demodulated signal with equal gain responsive tothe subcarrier power ratio P_(c) _(—) _(R), having a reduced error rateresulting from the diversity effect of the two demodulating paths A andB.

As described above, like the first embodiment, the second embodiment isstructured so that subcarrier component power values P_(c) _(—) _(A) andP_(c) _(—) _(B) corresponding to (CNR)_(A) and (CNR)_(B) are calculatedafter the Fourier transform, and adaptive combining diversity is carriedout using the result. This structure makes it possible to carry outadaptive combining diversity without being affected by channelestimation errors, thus improving the receiving performance of thediversity receiver.

THIRD EMBODIMENT

The diversity receivers in the first and second embodiments arestructured so that estimated power or subcarrier power is determinedfrom frequency domain signals output from the FFT units 42 and 52, basedon which adaptive combining diversity is carried out. In the thirdembodiment, the power levels of the signals input through the antennas11, 21 are determined and adaptive combining diversity is carried out byusing these power levels, as described below.

FIG. 6 is a block diagram showing the diversity receiver in the thirdembodiment.

The structure of the diversity receiver in FIG. 6 is the same as shownin FIG. 1 in the first embodiment or in FIG. 6 in the second embodiment,except for the first OFDM demodulator 15 b, second OFDM demodulator 25b, power ratio comparator 31 b, input connections to the AGC units 13,23, and output connections from the ADC's 14, 24.

The operation of the diversity receiver in the third embodiment will bedescribed below. Descriptions of structures that are the same as in thefirst or second embodiment will be omitted.

The first gain detector 47 in the first OFDM demodulator 15 b receives afirst received signal from the first ADC 14, calculates a differencebetween the average power of the first received signal and a desiredpower value, and outputs the calculated result as a first power controlsignal to the power ratio comparator 31 b and the first AGC unit 13.Similarly, the second gain detector 57 in the second OFDM demodulator 25b receives a second received signal from the second ADC 24, calculatesthe difference between the average power of the second received signaland a desired power value, and outputs the calculated result as a secondpower control signal to the power ratio comparator 31 b and the secondAGC unit 23.

The first power control signal and the second power control signal areused in the first AGC unit 13 and second AGC unit 23 to select thedegree of amplification of the signals received through the antennas 11and 21; a higher signal level of the power control signal indicates alower antenna output signal power.

In the gain detectors 47 and 57, increasing the period of time overwhich the power of the received signal is averaged can improve thereliability of the final average value by allowing errors due to randomnoise to cancel out. If the averaging period is too long, however, timevariations in the power of the received signal can cause performancedegradation in some applications. The period of time over which thereceived signal power is averaged should therefore be optimized for eachapplication.

The first received signal and the second received signal are input tothe OFDM demodulators 15 b, 25 b after gain adjustment. Accordingly, ifthere is a difference between the antenna gains on demodulation paths Aand B, for example, a difference in the power levels of the signalsreceived through antennas 11, 12 causes a difference in the noise powerof the first received signal and the second received signal.

The difference in noise power affects the carrier-to-noise ratio of thesignal output from the selective/equal gain combining selector 33. Inparticular, when one of the received signals on the demodulation paths Aand B is weaker than the other, and accordingly the amplification factorof the AGC unit 13 or 23 must be increased, the diversity effect isreduced. In order to prevent the diversity effect from being reduced,therefore, it is effective to control adaptive combining diversityaccording to the power ratio calculated by using the received signalsbefore their gains are adjusted in the AGC units 13, 23.

The power ratio comparator 31 b receives the first control signal outputfrom the first gain detector 47, the second control signal output fromthe second gain detector 57, and a predetermined threshold value. In thethird embodiment, the predetermined threshold value, which is comparedwith the power ratio obtained from the above-mentioned power values,will be referred to as the power ratio threshold value, as in the firstembodiment and the second embodiment.

By using the first power control signal and the second power controlsignal output from the gain detectors 47, 57, the power ratio comparator31 b determines whether the first received signal or the second receivedsignal has the higher power level. Then the power ratio comparator 31 buses the above two power control signals to calculate received signalpowers P_(A), P_(B) corresponding to the power control signals, comparesthe received signal power ratio P_(R), which is obtained by dividing thelarger one of the two received signal powers P_(A), P_(B) by the smallerone, with the power ratio threshold value, and outputs to theselective/equal gain combining selector 33, for each subcarrier, asignal varying responsive to the comparison result.

More specifically, when the received signal power ratio P_(R) is smallerthan the power ratio threshold value, the power ratio comparator 31 bsends the selective/equal gain combining selector 33 a signal indicatingthat the demodulated signal obtained in the equal-gain signal combiner62 is to be output. Conversely, when the received signal power ratioP_(R) is larger than the power ratio threshold value, the power ratiocomparator 31 b sends the selective/equal gain combining selector 33 asignal indicating that the demodulated signal corresponding to thelarger of the two received signal power values P_(c) _(—) _(A), P_(c)_(—) _(B) is to be selected.

Based on the signal received from the power ratio comparator 31 b, theselective/equal gain combining selector 33 selects either selectiondiversity, in which either the demodulated signal from the first OFDMdemodulator 15 b or the demodulated signal from the second OFDMdemodulator 25 b is selected and output, or equal gain combiningdiversity, in which a demodulated signal obtained by combining the twodemodulated signals from the first OFDM demodulator 15 b and second OFDMdemodulator 25 b with equal gain is selected and output.

That is, according to the output of the power ratio comparator 31 b, theselective/equal gain combining selector 33 outputs the output signalfrom the first demodulator 46 alone, the output signal from the seconddemodulator 56 alone, or a combined output signal obtained by combiningthe above output signals.

Accordingly, the selectively combined or equal gain combined signaloutput from the selective/equal gain combining selector 33 is ademodulated signal obtained for each subcarrier component by adaptivelyselecting either a demodulated signal obtained by equal gain combiningdiversity responsive to the received signal power ratio P_(R) or ademodulated signal obtained by selecting one of the two demodulatedsignals corresponding to the received signals, having a reduced errorrate resulting from the diversity effect of the two demodulating paths Aand B.

As described above, since the diversity receiver in the third embodimentis structured so that adaptive combining diversity is carried out byusing the control signals for adjusting the power levels of signalsreceived through the antennas 11, 21, even if there is a differencebetween the receiving power levels of the two received signals, it ispossible to combine the signals without reducing the diversity effect.The performance of the diversity receiver can also be improved.Furthermore, even if the receiving power levels of the two receivedsignals differ from each other, it is also possible to combine thesignals without reducing the diversity effect, resulting in improvedreceiving performance of the receiver.

In the third embodiment, the received signal power of each signal iscalculated by using the two power control signals as described above anda signal is output from the power ratio comparator 31 b on the basis ofthe received signal power, but the signal output from the power ratiocomparator 31 b may be based directly on the power control signals. Inthis case, since as the antenna output signal power decreases, the powercontrol signal power increases, as mentioned above, it is necessary toregard a higher level of the power control signal as indicating areduced carrier-to-noise ratio. To determine the power control signalratio, therefore, the reciprocal ratio of the power control signalvalues is determined, and adaptive combining diversity is carried outaccording to the reciprocal ratio. When the output of the signalselector 61 is used as the selectively combined or equal gain combinedsignal, it is necessary to choose between the output signals from thedemodulators 46, 56 in the OFDM demodulators 15 b, 25 b by selecting theoutput signal corresponding to the smaller of the two power controlsignals.

FOURTH EMBODIMENT

The diversity receiver in the third embodiment is structured so as todetermine the power levels of the signals received through the antennas11, 21 and carry out adaptive combining diversity per OFDM symbol byusing these power levels. The diversity receiver in the fourthembodiment carries out adaptive combining diversity by using the powerlevels of the signals received through the antennas 11, 21 and thesignal power derived from channel estimation for each subcarrier, asdescribed below.

FIG. 7 is a block diagram showing the diversity receiver in the fourthembodiment.

The structure of the diversity receiver in FIG. 7 is the same as shownin FIG. 6 in the third embodiment, except for the first OFDM demodulator15 c, second OFDM demodulator 25 c, and power ratio comparator 31 c, andexcept for the output connections to the power ratio comparator 31 cfrom the estimated power value calculators 44, 54, which are the same asshown in FIG. 1 in the first embodiment.

Next, the operation of the diversity receiver will be described.Descriptions of structures that are the same as in the first and thirdembodiments will be omitted.

The power ratio comparator 31 c receives the first control signal outputfrom the first gain detector 47, the second control signal output fromthe second gain detector 57, the first estimated power output from thefirst estimated power value calculator 44, the second estimated poweroutput from the second estimated power value calculator 54, and apredetermined threshold value. In the fourth embodiment, as in the firstand third embodiments, the predetermined threshold value will bereferred to as the power ratio threshold value.

From the first power control signal, the power ratio comparator 31 ccalculates a coefficient by which to multiply the first estimated powervalue. Similarly, from the second power control signal it calculates acoefficient by which to multiply the second estimated power value. Thepower ratio comparator 31 c also determines which of the estimated powervalues, thus multiplied, is larger. The power ratio comparator 31 cfurther compares the power ratio threshold value with a value obtainedby dividing the larger one of the two multiplication results, obtainedby multiplying the estimated power values by the correspondingcoefficients, by the smaller one, and outputs a signal varying for eachsubcarrier responsive to the comparison result to the selective/equalgain combining selector 33.

Responsive to the signal received from the power ratio comparator 31 a,the selective/equal gain combining selector 33 selects either selectiondiversity, in which either the demodulated signal from the first OFDMdemodulator 15 c or the demodulated signal from the second OFDMdemodulator 25 c is selected and output, or equal gain combiningdiversity, in which a demodulated signal obtained by combining the twodemodulated signals from the first OFDM demodulator 15 c and second OFDMdemodulator 25 c with equal gain is selected and output.

The coefficients by which the outputs of the first estimated power valuecalculator 44 and second estimated power value calculator 54 aremultiplied will now be described. As noted above, too large a noisepower differential between the first received signal and the secondreceived signal reduces the diversity effect. In order to prevent thediversity effect from being reduced, therefore, it is effective tocontrol adaptive combining diversity by considering the power ratio ofthe received signals before their gains are adjusted.

The relationship among the power values of the signals received throughthe first antenna 11 and the second antenna 21, the gain adjustmentquantities of the signals received through the first antenna 11 and thesecond antenna 21, and the values of the output signals of the firstestimated power value calculator 44 and second estimated power valuecalculator 54 corresponding to a subcarrier component can beapproximately represented by equation 6 below.

$\begin{matrix}{\frac{P_{A}}{P_{B}} = \frac{G_{B}x_{A}}{G_{A}x_{B}}} & (6)\end{matrix}$

In this equation, P_(A) is the power of the signal received through thefirst antenna 11, P_(B) is the power of the signal received through thesecond antenna 21, G_(A) is the gain adjustment quantity of the signalreceived through the first antenna 11, G_(B) is the gain adjustmentquantity of the signal received through the second antenna 21, x_(A) isthe output of the first estimated power value calculator 44corresponding to the subcarrier component, and x_(B) is the output ofthe second estimated power value calculator 54 corresponding to thesubcarrier component.

From the above equation, it can be seen that gain adjustment of theoutput of the first estimated power value calculator 44 in the first AGCunit 13 may be carried out by multiplying the output of the firstestimated power value calculator 44 by a coefficient proportional toG_(B). Similarly, gain adjustment of the output from the secondestimated power value calculator 54 in the second AGC unit 23 may becarried out by multiplying the output of the second estimated powervalue calculator 54 by a coefficient proportional to G_(A).

The power ratio comparator 31 c carries out the decision processdescribed by equations 4 and 5, for example, responsive to each pair ofestimated power values obtained by multiplication by the above-mentionedcoefficients.

Responsive to the output from the power ratio comparator 31 c, theselective/equal gain combining selector 33 outputs the first modulatedsignal, the second modulated signal, or a modulated signal obtained bycombining the first modulated signal and the second modulated signalwith equal gain.

Accordingly, the output of the selective/equal gain combining selector33 is a signal obtained by carrying out adaptive combining diversityresponsive to the power ratio of the signals received through the twoantennas 11, 21 and the power ratio corresponding to the result ofchannel characteristic estimation of the received signals after the gainadjustment.

As described above, since the fourth embodiment provides a structure inwhich adaptive combining diversity is carried out using the powercontrol signals for adjusting the power levels of the signals receivedfrom antennas 11, 21 and the power values corresponding to the resultsof channel characteristic estimation for each subcarrier component, evenif there is a difference between the power levels of the two receivedsignals, it is possible to combine the signals without reducing thediversity effect, resulting in improved receiving performance of thereceiver.

FIFTH EMBODIMENT

The diversity receiver in the fourth embodiment is structured so thatadaptive combining diversity is carried out by using the power levels ofthe signals received through the antennas 11, 21 and the power valuescorresponding to the results of the channel characteristic estimationfor each subcarrier. The diversity receiver in the fifth embodimentcarries out adaptive combining diversity by using the power levels ofthe signals received through the antennas 11, 21 and the signal powerfor each subcarrier, as described below.

FIG. 8 is a block diagram showing the diversity receiver in the fifthembodiment.

The structure of the diversity receiver in FIG. 8 is the same as shownin FIG. 7 in the fourth embodiment, except for the first OFDMdemodulator 15 d, second OFDM demodulator 25 d, power ratio comparator31 d, first subcarrier power calculator 45, and second subcarrier powercalculator 55, and except that there are no output connections to thepower ratio comparator 31 d from the first estimated power valuecalculator 44 and second estimated power value calculator 54. The firstsubcarrier power calculator 45 and second subcarrier power calculator 55are the same as shown in FIG. 5 in the second embodiment.

Next, the operation of the diversity receiver in the fifth embodimentwill be described. Descriptions of structures that are the same as inthe first and fourth embodiments will be omitted.

The power ratio comparator 31 d receives the first control signal outputfrom the first gain detector 47, the second control signal output fromthe second gain detector 57, the first subcarrier power output from thefirst subcarrier power calculator 45, the second subcarrier power outputfrom the second subcarrier power calculator 55, and a predeterminedthreshold value. In the fifth embodiment, as in the first and fourthembodiments, the predetermined threshold value, which is compared with apower ratio obtained from the above-mentioned power values, will bereferred to as the power ratio threshold value.

The power ratio comparator 31 d multiplies the first subcarrier power bya coefficient determined from the first power control signal. Similarly,it multiplies the second subcarrier power by a coefficient determinedfrom the second power control signal. The power ratio comparator 31 dfurther determines which of the multiplication results for the firstsubcarrier power and second subcarrier power is larger; then it comparesthe power ratio threshold value with a value obtained by dividing thelarger one of the two multiplication results by the smaller one, andoutputs a signal varying for each subcarrier responsive to thecomparison result to the selective/equal gain combining selector 33. Theabove coefficient may be determined in the same way as in the fourthembodiment. More specifically, it can be determined by processing of theoutputs of the first estimated power value calculator 44 and secondestimated power value calculator 54 similar to the processing of theoutputs of the first subcarrier power calculator 45 and secondsubcarrier power calculator 55.

Responsive to the signal received from the power ratio comparator 31 d,the selective/equal gain combining selector 33 selects either selectiondiversity, in which either the demodulated signal from the first OFDMdemodulator 15 d or the demodulated signal from the second OFDMdemodulator 25 d is selected and output, or equal gain combiningdiversity, in which a demodulated signal obtained by combining the twodemodulated signals from the first OFDM demodulator 15 d and second OFDMdemodulator 25 d with equal gain is selected and output.

The power ratio comparator 31 d carries out the decision processdescribed by equations 4 and 5, for example, responsive to the resultsobtained by multiplying the subcarrier powers output from the firstsubcarrier power calculator 45 and second subcarrier power calculator 55by the coefficients.

Responsive to the output from the power ratio comparator 31 d, theselective/equal gain combining selector 33 outputs the first modulatedsignal, the second modulated signal, or a modulated signal obtained bycombining the first modulated signal and the second modulated signalwith equal gain.

Accordingly, the output of the selective/equal gain combining selector33 is a signal obtained by adaptively switching between equal gaincombining diversity and selection diversity for each subcarrierresponsive to the power ratio of the pair of signals received throughthe antennas 11, 21 and the subcarrier power ratio of the receivedsignals after the gain adjustment.

As described above, since the fifth embodiment provides a structure inwhich adaptive combining diversity is carried out using the powercontrol signals for adjusting the power levels of the signals receivedfrom the antennas 11, 21 and the power values of the subcarriercomponents after the Fourier transform, even if there is a differencebetween the power levels of the two received signals, it is possible tocombine the signals without reducing the diversity effect, resulting inimproved receiving performance of the receiver, and it is possible tocarry out adaptive combining diversity without being affected by channelcharacteristic estimation error, also resulting in improved receivingperformance of the receiver.

SIXTH EMBODIMENT

The diversity receiver in the fifth embodiment is structured so thatadaptive combining diversity is carried out by using the power levels ofthe signals received through the antennas 11, 21 and the signal power ofeach subcarrier. The diversity receiver in the sixth embodiment carriesout adaptive combining diversity by adaptively changing the thresholdvalue for the power ratio comparator responsive to the power levels ofthe signals received through the antennas 11, 21 and using the thresholdvalue and the estimated power values, as described below.

FIG. 9 is a block diagram showing the diversity receiver in the sixthembodiment.

The structure of the diversity receiver in FIG. 9 is the same as shownin FIG. 8 in the fifth embodiment, except for the power ratio comparator31 e, a threshold conversion table unit 32 provided between the firstgain detector 47 and second gain detector 57 and the power ratiocomparator 31 e, and except that the power ratio threshold value isoutput from the threshold conversion table unit 32 to the power ratiocomparator 31 e. The first OFDM demodulator 15 e and second OFDMdemodulator 25 e in FIG. 9 have the same structure as the first OFDMdemodulator 15 c and second OFDM demodulator 25 c in FIG. 7 in thefourth embodiment.

Next, the operation of the diversity receiver in the sixth embodimentwill be described. Descriptions of structures that are the same as inthe first and fifth embodiments will be omitted.

The threshold conversion table unit 32 outputs a power ratio thresholdvalue that varies responsive to the first power control signal outputfrom the first gain detector 47 and the second power control signaloutput from the second gain detector 57. That is, while the power ratiothreshold value is predetermined in the first to fifth embodiments, thethreshold conversion table unit 32 outputs a power ratio threshold valuevarying responsive to the first power control signal and the secondpower control signal.

The power ratio threshold value in this embodiment is determined fromequation 6 by multiplying a predetermined power ratio threshold value bythe ratio of the first power control signal and the second power controlsignal. Accordingly, the threshold conversion table unit 32 may prestorethe results of multiplication of the predetermined power ratio thresholdvalue by the ratio of the first control signal and the second controlsignal.

The power ratio comparator 31 e receives the first estimated powervalue, the second estimated power value, and the predetermined powerratio threshold value, and determines which of the first estimated powervalue and the second estimated value is larger; then it compares thepower ratio threshold value received from the threshold conversion tableunit 32 with a value obtained by dividing the larger one of the twoestimated power values by the smaller one, and outputs a signal varyingfor each subcarrier responsive to the comparison result to theselective/equal gain combining selector 33.

Responsive to the signal received from the power ratio comparator 31 e,the selective/equal gain combining selector 33 selects either selectiondiversity, in which either the demodulated signal from the first OFDMdemodulator 15 e or the demodulated signal from the second OFDMdemodulator 25 e is selected and output, or equal gain combiningdiversity, in which a demodulated signal obtained by combining the twodemodulated signals from the first OFDM demodulator 15 e and second OFDMdemodulator 25 e with equal gain is selected and output.

That is, the selective/equal gain combining selector 33 outputs,responsive to the output from the power ratio comparator 31 e, the firstmodulated signal, the second modulated signal, or a modulated signalobtained by combining the first modulated signal and the secondmodulated signal with equal gain.

Accordingly, the output of the selective/equal gain combining selector33 is a signal obtained by adaptively selecting either one of a pair ofdemodulated signals on the demodulation paths A, B, which is selectedresponsive to the power ratio of the two channel characteristicestimation values of the two received signals for each subcarriercomponent, or a demodulated signal obtained by combining the demodulatedsignals on demodulation paths A, B; the diversity effect of the twodemodulation paths A and B reduces the error rate of the modulatedsignal.

As described above, the sixth embodiment provides a structure in whichadaptive combining diversity is carried out by adaptively varying thepower ratio threshold value responsive to the power levels of thesignals received through the antennas 11, 21, and using the varyingpower ratio threshold value and power values corresponding to thechannel characteristic estimation results; thus it can eliminate theneed for a multiplier for correcting the power values resulting fromchannel estimation according to the power control signals, with theeffect that the diversity combining process for each subcarrier can becarried out by a receiver with less circuitry, without a reduction ofthe diversity effect due to a difference between the receiving powerlevels.

SEVENTH EMBODIMENT

The diversity receiver in the sixth embodiment carries out adaptivecombining diversity by adaptively changing the threshold valueresponsive to the power levels of the signals received through theantennas 11, 21 and using the threshold value and the estimated powervalues as described above. The diversity receiver in the seventhembodiment is another example of the diversity receiver in the sixthembodiment

FIG. 10 is a block diagram showing the diversity receiver in the seventhembodiment.

The structure of the diversity receiver in FIG. 10 is the same as shownin FIG. 9 in the sixth embodiment, except for the power ratio comparator31 f, first subcarrier power calculator 45, and second subcarrier powercalculator 55, and except that there are no connections from the firstestimated power value calculator 44 and second estimated power valuecalculator 54 to the power ratio comparator 31 f. The first subcarrierpower calculator 45 and second subcarrier power calculator 55 are thesame as in FIG. 5 in the second embodiment. The first OFDM demodulator15 f and second OFDM demodulator 25 f in FIG. 10 have the same structureas the first OFDM demodulator 15 d and second OFDM demodulator 25 d inFIG. 8 in the fifth embodiment.

Next, the operation of the diversity receiver in the seventh embodimentwill be described. Descriptions of structures that are the same as inthe first to sixth embodiments will be omitted.

The threshold conversion table unit 32 determines a power ratiothreshold value responsive to the first power control signal and thesecond power control signal as described in the sixth embodiment andoutputs it to the power ratio comparator 31 f.

The power ratio comparator 31 f compares the first subcarrier powervalue received from the first subcarrier power calculator 45 with thesecond subcarrier power value received from the second subcarrier powercalculator 55 and determines which of the two subcarrier power values islarger. It further compares a value obtained by dividing the larger oneof the two subcarrier power values by the smaller one with the powerratio threshold value received from the threshold conversion table unit32, and outputs a signal varying for each subcarrier responsive to thecomparison result to the selective/equal gain combining selector 33.

Responsive to the signal received from the power ratio comparator 31 f,the selective/equal gain combining selector 33 selects either selectiondiversity, in which either the demodulated signal from the first OFDMdemodulator 15 f or the demodulated signal from the second OFDMdemodulator 25 f is selected and output, or equal gain combiningdiversity, in which a demodulated signal obtained by combining the twodemodulated signals from the first OFDM demodulator 15 f and second OFDMdemodulator 25 f with equal gain is selected and output.

That is, the selective/equal gain combining selector 33 outputs,responsive to the output from the power ratio comparator 31 f, a signaloutput from the first demodulator 46 alone, a signal output from thesecond demodulator 56 alone, or a signal obtained by combining thesignals output from the first demodulator 46 and second demodulator 56with equal gain.

Accordingly, the output of the selective/equal gain combining selector33 is a demodulated signal obtained by adaptively selecting one of thepair of demodulated signals on the demodulation paths A, B, responsiveto the power ratio of the two channel characteristic estimation valuesof the two received signals for each subcarrier component, or ademodulated signal obtained by combining the demodulated signals ondemodulation paths A, B; the diversity effect of the two demodulationpaths A and B reduces the error rate of the modulated signal.

As described above, the seventh embodiment provides a structure in whichadaptive combining diversity is carried out by adaptively varying thepower ratio threshold value responsive to the power levels of signalsreceived through the antennas 11, 21, and using the varying power ratiothreshold value and power values corresponding to the channelcharacteristic estimation results; thus it can eliminate the need for amultiplier for correcting the power values resulting from the channelestimation according to the power control signals, so the diversitycombining process for each subcarrier can be carried out by a receiverwith less circuitry, without a reduction of the diversity effect due toa difference between the receiving power levels. It is so structuredthat adaptive combining diversity is carried out using the power valuesof the subcarrier components after the Fourier transform, making itpossible to carry out adaptive combining diversity without beingaffected by channel estimation error, resulting in improved receivingperformance of the receiver.

EIGHTH EMBODIMENT

The diversity receiver in the seventh embodiment carries out adaptivecombining diversity by adaptively varying a threshold value in the powerratio comparator responsive to the power levels of the signals receivedthrough the antennas 11, 21 and using the threshold value together withthe signal power values of the subcarriers. In the diversity receiver inthe eighth embodiment, in addition to the power level and the estimatedpower value P_(es) used in the sixth embodiment, an error count obtainedas a result of the correction of errors in the demodulated signalsoutput from the first demodulator 46 and second demodulator 56 is alsotaken into account to carry out adaptive combining diversity, asdescribed below.

In general, a received signal using a Reed-Solomon error correcting coderequires a Reed-Solomon demodulator in the error corrector of thereceiver. A Reed-Solomon demodulator performs error correction for thereceived signal by using parity information added to the received datapacket to reproduce the received data. The received data stream isdivided into blocks of a given size and parity information is insertedin each block, the data and parity information constituting a datapacket with a given amount of data. The Reed-Solomon demodulatorcorrects errors in each data packet independently.

If the number of errors in a data packet exceeds the error correctingcapability of the parity information, the Reed-Solomon demodulatorbecomes unable to perform error correction, but it can still count thenumber of data packets with errors that were uncorrectable. Accordingly,it is possible to set the Reed-Solomon demodulator to output demodulatedsignals and also to output the number of data packets with uncorrectableerrors at regular intervals. The diversity receiver in this embodimentmakes use of this count of the number of data packets with uncorrectableerrors. In the following descriptions, the number of data packets withuncorrectable errors will be represented as N_(ep). It will be assumedthat the number of errors is the same as the number of data packets withuncorrectable errors.

FIG. 11 is a block diagram showing a diversity receiver in the eighthembodiment. The structure of the diversity receiver in FIG. 11 is thesame as shown in FIG. 9 in the sixth embodiment, except for the powerratio comparator 31 g, a first pre-combination error correction unit 63,and a second pre-combination error correction unit 64, and except thatthere is no threshold conversion table unit. The first pre-combinationerror correction unit 63 and second pre-combination error correctionunit 64 operate following the first demodulator 46 and seconddemodulator 56 so that adaptive combining diversity is carried out byusing the counts of the number of data packets with uncorrectableerrors. FIG. 12 is a block diagram showing the structure of the firstpre-combination error correction unit 63 and second pre-combinationerror correction unit 64 in FIG. 11; a counter 66 in FIG. 12 counts thenumber of data packets with uncorrectable errors output from theReed-Solomon demodulator 65.

The Reed-Solomon decoder 65 in the first pre-combination errorcorrection unit 63 corrects errors in the first demodulated signaloutput from the first demodulator 46, and outputs a signal indicatingthe number of data packets with uncorrectable errors N_(ep) _(—) _(A)occurring in a predetermined period of time, which will be referred toas an uncorrectable error signal below. Responsive to the uncorrectableerror signal, the counter 66 calculates the number N_(ep) _(—) _(A) ofdata packets with uncorrectable errors in the first demodulated signaland outputs a signal indicating the calculated result to the power ratiocomparator 31 g.

Similarly, the Reed-Solomon decoder 65 in the second pre-combinationerror correction unit 64 corrects errors in the second demodulatedsignal output from the second demodulator 56, and outputs anuncorrectable error signal. Responsive to the uncorrectable errorsignal, the counter 66 calculates the number N_(ep) _(—) _(B) of datapackets with uncorrectable errors in the second demodulated signal andoutputs a signal indicating the calculated result to the power ratiocomparator 31 g. In the following descriptions of the eighth embodiment,the uncorrectable error signal output from the first pre-combinationerror correction unit 63 will be referred to as the first uncorrectableerror signal; the uncorrectable error signal output from the secondpre-combination error correction unit 64 will be referred to as thesecond uncorrectable error signal. Similarly, the number of data packetswith uncorrectable errors in the first demodulated signal N_(ep) _(—)_(A) will be referred to as the first number of data packets withuncorrectable errors N_(ep) _(—) _(A); the number of data packets withuncorrectable errors in the second demodulated signal N_(ep) _(—) _(B)will be referred to as the second number of data packets withuncorrectable errors N_(ep) _(—) _(B).

The power ratio comparator 31 g receives the first power control signaloutput from the first gain detector 47, the second power control signaloutput from the second gain detector 57, the first estimated power valueP_(es) _(—) _(A) output from the first estimated power value calculator44, the second estimated power value P_(es) _(—) _(B) output from thesecond estimated power value calculator 54, the first uncorrectableerror signal, and the second uncorrectable error signal.

A first threshold value Th₁ for the first power control signal andsecond power control signal, a second threshold value Th₂ for the firstuncorrectable error signal and second uncorrectable error signal, and athird threshold value Th₃ for the first estimated power value P_(es)_(—) _(A) and the second estimated power value P_(es) _(—) _(B) areinput in advance to the power ratio comparator 31 g.

FIG. 13 is a flow-diagram showing an example of the operation of thepower ratio comparator 31 g in the diversity receiver in FIG. 11.

In the power ratio comparator 31 g in FIG. 11, the average power valueof the first received signals is calculated from the first power controlsignal, and the average power value of the second received signal iscalculated from the second power control signal (S1). Then thedifference ΔP between the two average power values is calculated (S2),and the difference ΔP is compared with the first threshold value Th₁(S3).

As a result of the comparison, if the difference ΔP between the averagepower values is larger than the first threshold value Th₁ (S4: Yes), thefirst number of data packets with uncorrectable errors N_(ep) _(—) _(A),indicated by the first uncorrectable error signal, and the second numberof data packet with uncorrectable errors N_(ep) _(—) _(B), indicated bythe second uncorrectable error signal, are compared with the secondthreshold value Th₂ (S5).

If the result of step S5, is that the first number N_(ep) _(—) _(A) isfound to be smaller than the second threshold value Th₂ (S6: Yes), andthe second number N_(ep) _(—) _(B) is found to be larger than the secondthreshold value Th₂ (S7: Yes), the power ratio comparator 31 g outputs asignal indicating that the first demodulated signal should be selectedby the signal selector 61 in the selective/equal gain combining selector33 (S8).

If the result of step S5 is that the first number N_(ep) _(—) _(A) isfound to be larger than the second threshold value Th₂ (S6: No and S9:Yes), and the second number N_(ep) _(—) _(B) is found to be smaller thanthe second threshold value Th₂ (S10: Yes), the power ratio comparator 31g outputs a signal indicating that the second demodulated signal shouldbe selected by the signal selector 61 in the selective/equal gaincombining selector 33 (S11).

In other words, in steps S5 to S11, if it is determined that just one ofthe first number of data packets with uncorrectable errors, indicated bythe first uncorrectable error signal N_(ep) _(—) _(A), or the secondnumber of data packets with uncorrectable errors, indicated by thesecond uncorrectable error signal N_(ep) _(—) _(B), is larger than thesecond threshold value Th₂, a signal is output from the power ratiocomparator 31 g to the selective/equal gain combining selector 33indicating that the demodulated signal in which the number of datapackets with uncorrectable errors is smaller than the second thresholdvalue Th₂ is to be output.

In other cases, that is, if the difference ΔP between the average powervalues is smaller than the first threshold value Th₁ (S4: No), if boththe first number of data packets with uncorrectable errors N_(es) _(—)_(A) and the second number of data packets with uncorrectable errorsN_(es) _(—) _(B) are smaller than the second threshold value Th₂ (S7:No), or if both the first number of data packets with uncorrectableerrors N_(es) _(—) _(A) and the second number of data packets withuncorrectable errors N_(es) _(—) _(B) are larger than the secondthreshold value Th₂ (S10: No), the power ratio comparator 31 gdetermines the estimated power ratio P_(es) _(—) _(R) from the firstestimated power value P_(es) _(—) _(A) and the second estimated powervalue P_(es) _(—) _(B), as represented by equation 7 below.

$\begin{matrix}{P_{es\_ R} = \frac{\max\left\lbrack {P_{es\_ A},P_{es\_ B}} \right\rbrack}{\min\left\lbrack {P_{es\_ A},P_{es\_ B}} \right\rbrack}} & (7)\end{matrix}$

In equation 7, max[X1, X2] is a function for selecting and outputtingthe larger one of X1 and X2; min[X1, X2] is a function for selecting andoutputting the smaller one of X1 and X2.

In this specific case, for example, the power ratio comparator 31 gdetermines which of the first estimated power value P_(es) _(—) _(A) andthe second estimated power value P_(es) _(—) _(B) is larger, and obtainsthe estimated power ratio P_(es) _(—) _(R) by dividing the larger one ofthe estimated power values by the smaller one. The power ratiocomparator 31 g further compares the obtained estimated power ratioP_(es) _(—) _(R) with the third threshold value Th₃ (S12).

If the result of the comparison in step S12 is that the estimated powerratio P_(es) _(—) _(R) is smaller than the third threshold value Th₃(S13: Yes), the power ratio comparator 31 g outputs to theselective/equal gain combining selector 33 a signal indicating that thecombined demodulated signal obtained in the equal-gain signal combiner62 is to be output for each subcarrier (S14).

If the result of the comparison in Step 12 is that the estimated powerratio P_(es) _(—) _(R) is larger than the third threshold value Th₃(S13: No), the power ratio comparator 31 g outputs to theselective/equal gain combining selector 33 a signal indicating that theselected demodulated signal obtained in the signal selector 61 is to beoutput for each subcarrier (S15).

Responsive to the signal received from the power ratio comparator 31 g,the selective/equal gain combining selector 33 outputs the demodulatedsignal obtained in the signal selector 61 or the equal-gain signalcombiner 62 to the error correction unit 34.

As described above, the eighth embodiment provides a structure in whichadaptive combining diversity is carried out responsive to the number ofdata packets with uncorrectable errors obtained from error correction ofthe demodulated signals output from the first demodulator 46 and thesecond demodulator 56, so the diversity combining process for eachsubcarrier can be carried out by a receiver with less circuitry, withouta reduction of the diversity effect due to a difference between thereceived power levels.

NINTH EMBODIMENT

The diversity receiver in the eighth embodiment carries out adaptivecombining diversity by taking account of the number of errors found byerror correction of the demodulated signals output from the firstdemodulator 46 and second demodulator 56, in addition to the powerlevels and estimated power values P_(es) _(—) _(A), P_(es) _(—) _(B).The diversity receiver in the ninth embodiment carries out adaptivecombining diversity by using, in addition to the power levels andestimated power values P_(es) _(—) _(A), P_(es) _(—) _(B), the number oferrors (number of data packets with uncorrectable errors) found by errorcorrection of either the first demodulated signal output from the firstdemodulator 46 or the second demodulated signal output from the seconddemodulator 56, and the number of errors (number of data packets withuncorrectable errors) found by error correction of the signal outputfrom the selective/equal gain combining selector 33, as described below.

FIG. 14 is a block diagram showing the structure of a diversity receiverin the ninth embodiment. The structure of the pre-combination errorcorrection unit 67 and the error correction unit 34 in FIG. 14 may bethe same as the structure of the first pre-combination error correctionunit 63 and second pre-combination error correction unit 64 in FIG. 12in the eighth embodiment. In the descriptions below, descriptions ofstructures that are the same as in the first to eighth embodiments willbe omitted.

The pre-combination error correction unit 67 performs error correctionof the first demodulated signal output from the first demodulator 46,and outputs to the power ratio comparator 31 h a third uncorrectableerror signal indicating a third number of data packets withuncorrectable errors N_(ep) _(—) _(pre) obtained in a predeterminedperiod of time. The error correction unit 34 performs error correctionof the selectively combined or equal gain combined signal, and outputsto the power ratio comparator 31 h a fourth uncorrectable error signalindicating a fourth number of data packets with uncorrectable errorsN_(ep) _(—) _(f).

The power ratio comparator 31 h receives the first power control signaloutput from the first gain detector 47, the second power control signaloutput from the second gain detector 57, the first estimated power valueoutput from the first-estimated power value calculator 44, the secondestimated power value output from the second estimated power valuecalculator 54, the third uncorrectable error signal output from thepre-combination error correction unit 67, and the fourth uncorrectableerror signal output from the error correction unit 34.

A first threshold value Th₁ for the first power control signal and thesecond power control signal, a fourth threshold value Th₄ for the thirduncorrectable error signal, a fifth threshold value Th₅ for the fourthuncorrectable error signal, and a sixth threshold value Th₆ for thefirst estimated power value and the second estimated power value areinput in advance to the power ratio comparator 31 h.

FIG. 15 is a flow diagram showing an example of the operation of thepower ratio comparator 31 h of the diversity receiver in FIG. 14.

The power ratio comparator 31 h in FIG. 14 calculates the average powervalue of the first received signal from the input first power controlsignal and the average power value of the second received signal fromthe second power control signal (S21), calculates the difference ΔPbetween the two average power values (S22), and compares the differenceΔP with the first threshold value Th₁ (S23).

As a result of the comparison, if the difference ΔP between the averagepower values is larger than the first threshold value Th₁ (S24: Yes),the power ratio comparator 31 h compares the third number of datapackets having uncorrectable errors N_(ep) _(—) _(pre) indicated by thethird uncorrectable error signal with the fourth threshold value Th₄,and compares the fourth number of data packets having uncorrectableerrors N_(ep) _(—) _(B) indicated by the fourth uncorrectable errorsignal with the fifth threshold value Th₅ (S25).

If the result of step S25 is that the third number of data packetshaving uncorrectable errors N_(ep) _(—) _(pre) indicated by the thirduncorrectable error signal is found to be smaller than the fourththreshold value Th₄ (S26: Yes), and the forth number of data packetshaving uncorrectable errors N_(ep) _(—) _(f) indicated by the fourthuncorrectable error signal is found to be larger than the fifththreshold value Th₅ (S27: Yes) the power ratio comparator 31 h comparesthe average power values of the first and second received signalsobtained in step S21 and determines which average power value is thelarger (S28).

If the result of step S28 is that the first average power is larger thanthe second average power (S28: Yes), the power ratio comparator 31 houtputs a signal indicating that the first demodulated signal should beselected by the signal selector 61 in the selective/equal gain combiningselector 33 and the first demodulated signal should be output from theselective/equal gain combining selector 33 (S29) If the result of stepS28 is that the first average power value is smaller than the secondaverage power value (S28: No), the power ratio comparator 31 h outputs asignal indicating that the second demodulated signal should be selectedby the signal selector 61 in the selective/equal gain combining selector33 and should be output from the selective/equal gain combining selector33 (S30).

In other words, in steps S25 to S30, if it is determined that the thirdnumber of data packets with uncorrectable errors indicated by the thirduncorrectable error signal is smaller than the fourth threshold valueTh₄ and the fourth number of data packets with uncorrectable errorsindicated by the fourth uncorrectable error signal is larger than thefifth threshold value Th₅, the power ratio comparator 31 h selects thelarger of the average power values, and a signal is output from thepower ratio comparator 31 h to the selective/equal gain combiningselector 33, indicating that the demodulated signal, on eitherdemodulation path A or B, with the selected average power value is to beselected and output.

In other cases, that is, if the difference ΔP between the average powervalues is smaller than the first threshold value Th₁ (S24: No), if thethird number of data packets with uncorrectable errors N_(ep) _(—)_(pre) is equal to or larger than the fourth threshold value Th₄ (S26:No), or if the fourth number of data packets with uncorrectable errorsN_(ep) _(—) _(f) is equal to or less than the fifth threshold value Th₅(S27: No), the power ratio comparator 31 h determines the estimatedpower ratio P_(es) _(—) _(R) from the first estimated power value P_(es)_(—) _(A) and the second estimated power value P_(es) _(—) _(B) as inequation 7 in the eighth embodiment, and compares the estimated powerratio P_(es) _(—) _(R) with the sixth threshold value Th₆ (S32).

If the result of the comparison in step S32 is that the estimated powerratio P_(es) _(—) _(R) is found to be smaller than the sixth thresholdvalue Th₆ (S33: Yes), a signal is output from the power ratio comparator31 h to the selective/equal gain combining selector 33, indicating thatthe demodulated signal obtained in the equal-gain signal combiner 62should be output (S34).

For each subcarrier, if the result of the comparison in step S32 is thatthe estimated power ratio P_(es) _(—) _(R) is larger than the sixththreshold value Th₆ (S33: No), the power ratio comparator 31 h outputsto the selective/equal gain combining selector 33 a signal indicatingthat the demodulated signal obtained in the signal selector 61 is to beoutput (S35). Responsive to the signal received from the power ratiocomparator 31 h, the selective/equal gain combining selector 33 outputsthe demodulated signal obtained in the signal selector 61 or theequal-gain signal combiner 62 to the error correction unit 34.

As described above, the ninth embodiment provides a structure in whichadaptive combining diversity is carried out by using a third number ofdata packets with uncorrectable errors obtained from the result of errorcorrection in either the first demodulated signal or the seconddemodulated signal and a fourth number of data packets withuncorrectable errors obtained from the result of error correction of thesignal output from the selective/equal gain combining selector 33, inaddition to the power levels and the estimated power ratio P_(es), so adiversity combining process responsive to the number of errors on thedemodulation paths A and B for each subcarrier can be carried out by areceiver with less circuitry, without a reduction of the diversityeffect due to a difference between the received power levels.

In the eighth and ninth embodiments, the diversity process utilizes thenumber of data packets with uncorrectable errors, but it is alsopossible to use, together with the number of data packets withuncorrectable errors, an uncorrectable error packet ratio (also referredto as an error rate) obtained by dividing the number of data packetswith uncorrectable errors by the number of data packets received in thepredetermined period of time.

The first to ninth embodiments have a structure with two demodulationpaths, but the invention is not limited to two demodulation paths: thestructure can be easily adapted to the case in which switching betweenselection diversity and equal gain combining diversity is carried out ina diversity receiver with three or more demodulation paths.

In the third to ninth embodiments, the first gain detector 47 and thesecond gain detector 57 are disposed in the first OFDM demodulator andthe second OFDM demodulator, respectively, but they may be disposedoutside the two OFDM demodulators.

INDUSTRIAL APPLICABILITY

As described above, the diversity receiving method of the presentinvention is adapted to switch adaptively between selection diversityand equal gain combining diversity for each subcarrier responsive to thepower of the received signals on each of the demodulation paths, so incomparison with conventional diversity receiving methods that performonly selection or only equal gain combining, the diversity effect can beincreased and receiving performance can be improved, while in comparisonwith the practice of maximal ratio combining diversity, a diversityreceiver with a large diversity effect can be implemented in lesscircuitry.

1. A diversity receiver comprising: a plurality of demodulation pathsfor demodulating received signals and outputting demodulated signals; apower ratio comparator for calculating a power ratio from a first powercorresponding to a first received signal on one of the demodulationpaths and a second power corresponding to a second received signal onanother one of the demodulation paths, and comparing the power ratiowith a predetermined threshold value; a signal selector for selectingone of the demodulated signals output from the plurality of demodulationpaths and outputting the selected demodulated signal; an equal-gainsignal combiner for combining the demodulated signals output from theplurality of demodulation paths with predetermined gains, and outputtinga combined demodulated signal; a demodulated signal output unit foroutputting one of the demodulated signals, either the selecteddemodulated signal or the combined demodulated signal, responsive to aresult of the comparison in the power ratio comparator; and an estimatedpower value calculator that outputs, as said first power, an estimatedpower value obtained from the result of channel characteristicestimation using a reference signal contained in the first receivedsignal.
 2. The diversity receiver of claim 1, wherein the receivedsignals include a plurality of subcarrier components, and thedemodulated signal output unit outputs one of the demodulated signals,either the selected demodulated signal or the combined demodulatedsignal, for each subcarrier component.
 3. The diversity receiver ofclaim 1, wherein the threshold value used in the power ratio comparatoris determined from a condition that the received-power-to-noise-powerratio value of the demodulated signal obtained by combining theplurality of demodulated signals with equal gain equals a maximumreceived-power-to-noise-power ratio among thereceived-power-to-noise-power ratios of the plurality of demodulatedsignals.
 4. The diversity receiver of claim 3, wherein the demodulatedsignal output unit outputs either the demodulated signal obtained bycombining the plurality of demodulated signals with equal gain or theselected demodulated signal responsive to the power ratio and thethreshold value determined under said condition.
 5. The diversityreceiver of claim 3, wherein the signal selector selects a demodulatedsignal with a maximum received-power-to-noise-power ratio among thereceived-power-to-noise-power ratios of the demodulated signals outputfrom the demodulation paths.
 6. The diversity receiver of claim 1,wherein: the first received signal is an orthogonal frequency divisionmultiplexing (OFDM) signal modulated by an OFDM modulation system; andthe estimated power value calculator uses a pilot signal included in theOFDM signal as the reference signal.
 7. The diversity receiver of claim1, wherein the first received signal is an OFDM signal modulated by anOFDM modulation system, further comprising: a subcarrier powercalculator that outputs a subcarrier power of a subcarrier componentobtained by a Fourier transform of The OFDM signal, as said first power.8. The diversity receiver of claim 1, further comprising a gain detectorthat outputs a power control signal corresponding to a gain adjustmentquantity for adjusting said first power to a predetermined power level.9. The diversity receiver of claim 8, further comprising an estimatedpower value calculator That outputs an estimated power valuecorresponding to a result of channel characteristic estimation using areference signal contained in the first received signal as said firstpower, wherein: the power ratio comparator performs the comparison byusing a result of multiplication of the estimated power value by acoefficient determined by the gain adjustment quantity.
 10. Thediversity receiver of claim 9, further comprising: a subcarrier powercalculator that outputs a subcarrier power of a subcarrier componentobtained by a Fourier transform of the first received signal, the firstreceived signal being an OFDM signal, wherein: the power ratiocomparator uses a result of multiplication of the subcarrier power valueby a coefficient determined by the gain adjustment quantity as the firstpower.
 11. The diversity receiver of claim 9, Thither comprising athreshold conversion table unit that prestores, and outputs to the powerratio comparator, a threshold value corresponding to the gain adjustmentquantity.
 12. A diversity receiver, comprising: a plurality ofdemodulation paths for demodulating received signals and outputtingdemodulated signals; a power ratio comparator for calculating a powerratio from a first power corresponding to a fast received signal on oneof the demodulation paths and a second power corresponding to a secondreceived signal on another one of the demodulation paths, and comparingthe power ratio with a predetermined threshold value; a signal selectorfor selecting one of the demodulated signals output from the pluralityof demodulation pats and outputting the selected demodulated signal; anequal-gain signal combiner for combining the demodulated signals outputfrom the plurality of demodulation paths with predetermined gains, andoutputting a combined demodulated signal; and a demodulated signaloutput unit for outputting one of the demodulated signals, either theselected demodulated signal or the combined demodulated signal,responsive to a result of the comparison in the power ratio comparator;a gain detector that outputs a power control signal corresponding to again adjustment quantity for adjusting said first power to apredetermined power level; an estimated power value calculator thatoutputs an estimated power corresponding to a result of channelcharacteristic estimation using a reference signal contained in thefirst received signal; and a pre-combination error correction unit thatoutputs a number of errors or an error rate obtained as a result oferror correction of the demodulated signal output from said one of thedemodulation paths before it is input to the demodulated signal outputunit; wherein the power ratio comparator uses the power control signal.,the estimated power, and said number of errors or said error rate incomparing the power ratio with the predetermined threshold value.
 13. Adiversity receiver, comprising: a plurality of demodulation paths fordemodulating received signals and outputting demodulated signals; apower ratio comparator for calculating a power ratio from a first powercorresponding to a first received signal on one of the demodulationpaths and a second power corresponding to a second received signal onanother one of the demodulation pats, and comparing the power ratio witha predetermined threshold value; a signal selector for selecting one ofthe demodulated signals output from the plurality of demodulation pathsand outputting the selected demodulated signal; an equal-gain signalcombiner for combining the demodulated signals output from the pluralityof demodulation paths with predetermined gains, and outputting acombined demodulated signal; and a demodulated signal output unit foroutputting one of the demodulated signals, either the selecteddemodulated signal or the combined demodulated signal, responsive to aresult of the comparison in the power ratio comparator; a gain detectorthat outputs a power control signal corresponding to a gain adjustmentquantity for adjusting said first power to a predetermined power level;an estimated power value calculator that outputs an estimated powercorresponding to a result of channel characteristic estimation using areference signal contained in the first received signal, as said firstpower; a pre-combination error correction unit that outputs a number oferrors or an error rate obtained as a result of error correction of thedemodulated signal output from said one of the demodulation paths beforeit is input to the demodulated signal output unit; and an errorcorrection unit that outputs a number of errors or an error rateobtained as a result of error correction of the demodulated signaloutput from the demodulated signal output unit; wherein the power ratiocomparator uses the power control signal, the estimated power, thenumber of errors or the error rate output from the pre-combination errorcorrection unit, and the number of errors or the error rate output fromthe error correction unit in comparing the power ratio with thepredetermined threshold value.
 14. A diversity receiving methodincluding a plurality of demodulating processes for demodulating areceived signal and outputting a demodulated signal, comprising thesteps of: calculating a power ratio from a first power corresponding toa first received signal in one of the demodulation processes and asecond power corresponding to a second received signal in another one ofthe demodulation processes, and comparing the power ratio with a firstpredetermined threshold value; counting pre-combination errors todetermine a first error rate of the first received signal in said one ofthe demodulation processes and a second error rate of the secondreceived signal in said another one of the demodulation processes, andcomparing the first error rate and the second error rate with a secondpredetermined threshold value; selecting one of the demodulated signalsoutput from the plurality of demodulation processes and outputting theselected demodulated signal; combining the demodulated signals outputfrom the plurality of demodulation paths with predetermined gains, andoutputting a combined demodulated signal; and outputting one of thedemodulated signals, either the selected demodulated signal or thecombined demodulated signal, responsive to a result of the comparison inthe step of calculating and results of the comparisons in the step ofcounting pre-combination errors.
 15. The diversity receiving method ofclaim 14, further comprising: counting output errors in the output oneof the demodulated signals to determine a third error rate of thedemodulated signal; and comparing the third error rate with a thirdpredetermined threshold value; wherein the outputting of said one of thedemodulated signals is also responsive to a result of the comparisonperformed in the step of comparing the third error rate.
 16. A diversityreceiver comprising: a plurality of demodulation paths for demodulatingreceived signals and outputting demodulated signals; a power ratiocomparator for calculating a power ratio from a first powercorresponding to a first received signal on one of the demodulationpaths and a second power corresponding to a second received signal onanother one of the demodulation paths, and comparing the power ratiowith a predetermined threshold value; a pre-combination error correctionunit that outputs a number of errors or an error rate obtained as aresult of error correction of the demodulated signal output from saidone of the demodulation paths before it is input to the demodulatedsignal output unit; a signal selector for selecting one of thedemodulated signals output from the plurality of demodulation paths andoutputting the selected demodulated signal; an equal-gain signalcombiner for combining the demodulated signals output from the pluralityof demodulation paths with predetermined gains, and outputting acombined demodulated signal; and a demodulated signal output unit foroutputting one of the demodulated signals, either the selecteddemodulated signal or the combined demodulated signal, responsive to aresult of the comparison in the power ratio comparator and the number oferrors or error rate obtained by the pre-combination error correctionunit.
 17. The diversity receiver of claim 16, further comprising: anerror correction unit tat outputs a number of errors or an error rateobtained as a result of error correction of the demodulated signaloutput from the demodulated signal output unit; wherein the outputtingof the one of the demodulated signals in the demodulated signal outputunit is also responsive to the number of errors or error rate obtainedby the error correction unit.